1. Field
This application relates to drive coils that generate a magnetic field to supply power and operational commands to a remote receiving coil. This application also relates generally to E-class oscillators.
2. Related Art
Many applications require or would benefit from improved efficiency in L-C tank circuit oscillations. Achieving such efficiency, however, can be problematic. One problem is now presented in the context of an exemplary application involving BIOnic Neurons (BIONs). This problem as well as other can also be present in other applications.
BIONs include micro, electrical stimulators that can be implanted within a body. BION implants may be placed in or near nerves or muscles to be electrically stimulated or at other locations. BIONs may be elongated with metallic electrodes at each end that deliver electrical current to immediately surrounding biological tissues. The implantable electronic devices may be hermetically sealed with metallic electrodes attached thereto. They may contain electronic circuitry. BION implants may be about 100 times smaller in volume than conventional implantable electronic devices such as cardiac pacemakers and cochlear implants. Their small size can result in significant physical limits on power, data transmission and packaging.
Microelectronic circuitry and inductive coils that may control the electrical current applied to the electrodes may be protected from body fluids by a hermetically sealed capsule. The capsule may be covered with a biocompatible coating or sheath for further protection. The electronic circuitry may include an inductive coil, power storage capacitor and/or integrated circuit for performing various functions.
Upon command from an external component, the implanted BION may emit an electrical stimulation pulse that travels through the body tissues between and around its electrodes. This may activate, for example, local nerve fibers. This may be part of a treatment. The BION micro stimulator may receive power and control signals from an inductive coupling to an externally generated RF magnetic field. This may be used to recharge a BION's battery and to control the timing and parameters of stimulations generation by the BION. This may be achieved by inductive coupling of magnetic fields generated by extracorporeal antenna that do not require any electrical connections to the BIONs, as discussed in U.S. Pat. Nos. 5,193,539, 5,193,540, 5,324,316, 5,405,367, and 6,051,017, incorporated herein by reference. By selecting the appropriate strength and temporal patterning of stimulation, a desired therapeutic effect can be achieved.
The small, narrow shape of many BIONs may result in stringent requirements for wireless power, data transmission and the electromechanical assembly. Developing solutions to meet these requirements may be difficult.
For example, the inductive coupling between a primary inductive coil within an extracorporeal antenna utilized to power a BION and a small, secondary inductive coil within the BION itself may be difficult to establish and maintain within the stringent requirements of the BION's power, data transmission, and electromechanical assembly. One reason for this may be that the coefficient of inductive coupling between a large primary coil and a distant, small secondary coil across an air gap may be very low, e.g., less than 2%. Therefore, the BION may be assembled such that the length and the cross-sectional area of its receiving coil are maximized. However, the BION's small size may limit the BION's receiving coil size.
To compensate for a weak coupling coefficient, the strength of the primary RF magnetic field, generated by the extracorporeal antenna, for example, may be made high. However, excessive power dissipation may be undesirable. For example, the extracorporeal antenna may be driven to at least 200–400V or even at 500V in order to generate sufficient power to drive the remote, implanted BION. However, designing an appropriate oscillator to generate sufficient field strength can be problematic.
As is well understood in the art, power oscillators are often classified according to the relationship between the output voltage swing and the input voltage swing. It is often the design of the output stage that defines each class. Classification may be based on the amount of time the output devices operate during one complete cycle of signal swing. This may also be defined in terms of output bias current, or the amount of current flowing in the output with no applied signal.
Conventional A-Class amplifiers may not be efficient enough for field use, as they can exhibit significant power dissipation. An alternative choice is an E-Class amplifier, or E-Class oscillator. Class E operation may involve oscillators designed for rectangular input pulses, not sinusoidal waveforms. The output load may be a tuned circuit, with the output voltage resembling a damped single pulse. A Class-E oscillator may operate in a switched mode (ON or OFF) which can provide a very high collector efficiency that can theoretically approach 100%. In operation, the energy content, or drive level, of an inter-stage signal may be applied to a single RF transistor. In combination with a temperature-compensated bias circuit, the single transistor may be set so that the single RF transistor is always sufficiently driven ON or OFF with each cycle of the inter-stage signal, but is not overdriven ON or OFF.
The high field strength and low power dissipation requirements of some BION applications might be accomplished by using Class E amplification with a very high Q (>100) tuned circuit. However, it may be unclear how to effectively utilize a Class E oscillator in a BION application, where both power efficiency and data transmission are often needed.
These requirements can be in conflict, as power efficiency often requires highly resonant operation of the Class E oscillator, while data transmission often requires rapid modulation of the Class E oscillator. With respect to the rapid modulation, a problematic feature of the Class E oscillator, in BION applications, can be that both the position and duration of the drive pulse are critical. For a coil frequency of 2 MHz, any drive pulse over 125 ns can cause excessive power dissipation in the switch without significantly increasing the energy in the coil. However, producing various pulse widths can require additional components that increase the cost and size of the coil driver assembly, which can be impractical in BION applications.
Using a Class E oscillator in BION applications can cause additional problems. For example, the flexible shape of the BION may easily be deformed while it is worn by the patient. Such deformities can cause fluctuations in the inductance of the external coil, and a Class E oscillator may not inherently compensate for such fluctuations. Moreover, the electromechanical assembly requirements of BIONs can make it desirable to accommodate the driver circuitry on the coil itself. This type of construction can make a typical Class E oscillator unsuitable for BION applications. Further, complicated circuitry can be required to change pulse width for achieving desired AM modulation, when utilizing a typical Class E oscillator. Changes in pulse width can be undesirable because they can cause significant degradation of efficiency, something a battery-operated BION may have limited capacity to endure.
BIONs are an exemplary application. A number of other applications, such as radio communication, metal detectors, mine detection, or power and data transmission to many types of remote devices, may also benefit from an oscillator having an efficient driving mechanism.